1. Field of the Invention
This invention relates to cast steel materials for the manufacture of casings and pressure vessels for use in steam turbine plants for thermal electric power generation, and a method of making a pressure vessel (or cast steel article) by using such a cast steel material.
2. Description of the Related Art
As casings and pressure vessels for use in steam turbine plants for thermal electric power generation, cast steel articles are frequently used in order to accommodate their complicated shapes. The properties required for such cast steel articles are such that they have excellent high-temperature strength and high creep rupture strength because they are used at high temperatures, and that they have excellent weldability because any defects in such cast steel materials need to be repaired by welding.
Generally known materials useful for this purpose include CrMoV cast steel, 2.25%CrMo cast steel and CrMo cast steel. Among them, 2.25%CrMo cast steel and CrMo cast steel have excellent impact resistance at ordinary temperatures and, as a result, good weldability. However, since they contain no strengthening element such as V, they have insufficient creep rupture strength and hence fail to meet the requirements of a material for the manufacture of casings of steam turbines having yearly rising operating temperatures.
On the other hand, the aforesaid CrMoV cast steel has high creep rupture strength and high mechanical strength owing to its high carbon content. However, it has poor impact resistance and, as a result., poor weldability. Accordingly, this material has the disadvantage that it cannot be easily repaired by welding in the process for the manufacture of casings and pressure vessels.
An object of the present invention is to provide a cast steel material for pressure vessels which has improved impact resistance (weldability) and toughness while maintaining its creep rupture strength at a level equal to or higher than the excellent creep rupture strength currently possessed by CrMoV cast steel, as well as a method of making a pressure vessel (or cast steel article) by using this cast steel material for pressure vessels which permits a pressure vessel to be made without requiring a material working step such as forging.